1. Field of the Invention
The present invention relates to compositions and methods for providing mitochondria-selective targeting agents. Specifically, this invention focuses on compositions and methods of Gramicidin-S peptidyl conjugates for selective targeting of therapeutic agents to mitochondria. Furthermore, this invention focuses on compositions and methods of Gramicidin-S peptidyl TEMPO conjugates, particularly synthetic Gramicidin S-peptidyl TEMPO (2,2,6,6-tetramethylpiperidine-N-oxyl) conjugates.
2. Description of the Prior Art
Cells typically undergo some degree of oxidative stress by way of generating reactive oxygen species (“ROS”). Specifically, the cellular respiration pathway generates ROS within the mitochondrial membrane of the cell, see Kelso et al., Selective Targeting of a Redox-active Ubiquinone to Mitochondria within Cells: Antioxidant and Antiapoptotic Properties, J. OF BIOL. CHEM. 276:4588 (2001). Reactive oxygen species include free radicals, reactive anions containing oxygen atoms, and molecules containing oxygen atoms that can either produce free radicals or are chemically activated by them. Specific examples include superoxide anion, hydroxyl radical, and hydroperoxides.
Naturally occurring enzymes, such as superoxide dismutase (“SOD”) and catalase salvage ROS radicals to allow normal metabolic activity to occur.
Significant deviations from cell homeostasis, such as hemorrhagic shock, lead to an oxidative stress state, thereby causing “electron leakage” from the mitochondrial membrane. Said “electron leakage” produces an excess amount of ROS which the cell's natural antioxidants cannot compensate for. Specifically, SOD cannot accommodate the excess production of ROS associated with hemorrhagic shock which ultimately leads to premature mitochondria dysfunction and cell death via apoptosis, see Kentner et al., Early Antioxidant Therapy with TEMPOL during Hemmorhagic Shock Increases Survival in Rats, J. OF TRAUMA® INJURY, INFECTION, AND CRITICAL CARE, 968 (2002).
Cardiolipin (“CL”) is an anionic phospholipids exclusively found in the inner mitochondrial membrane of eukaryotic cells, see Iverson, S. L. and S. Orrenius, The cardiolipin-cytochrome c interaction and the mitochondrial regulation of apoptosis, ARCH. BIOCHEM. BiOPHYS. 423:37-46 (2003).
Under normal conditions, the pro-apoptotic protein cytochrome c is anchored to the mitochondrial inner membrane by binding with CL, see Tuominen, E. K. J., et al. Phospholipid cytochrome c interaction: evidence for the extended lipid anchorage, J. BIOL. CHEM., 277:8822-8826 (2002). The acyl moieties of CL are susceptible to peroxidation by reactive oxygen species. When ROS are generated within mitochondria in excess quantities, cytochrome c bound to CL can function as an oxidase and induces extensive peroxidation of CL in the mitochondrial membrane, see Kagan, V. E. et al., Cytochrome c acts as a cardiolipin oxygenase required for release of proapoptotic factors, NATURE CHEM. BIOL. 1:223-232 (2005); also Kagan, V. E. et al., Oxidative lipidomics of apoptosis: redox catalytic interactions of cytochrome c with cardiolipin and phosphatidylserine, FREE RAD. BIOL. MED. 37:1963-1985 (2005).
The peroxidation of the CL weakens the binding between the CL and cytochrome c, see Shidoji, Y. et al., Loss of molecular interaction between cytochrome c and cardiolipin due to lipid peroxidation, BIOCHEM. BIOPHYS. RES. COMM. 264:343-347 (1999). This leads to the release of the cytochrome c into the mitochondrial intermembrane space, inducing apoptotic cell death.
Further, the peroxidation of CL has the effect of opening the mitochondrial permeability transition pore “MPTP”), see Dolder, M. et al., Mitochondrial creatine kinase in contact sites: Interaction with porin and adenine nucleotide translocase, role in permeability transition and sensitivity to oxidative damage, BIOL. SIGNALS RECEPT., 10:93-111 (2001); also Imai, H. et al., Protection from inactivation of the adenine nucleotide translocator during hypoglycaemia-induced apoptosis by mitochondrial phospholipid hydroperoxide glutathione peroxidase, BIOCHEM. J., 371:799-809 (2003). Accordingly, the mitochondrial membrane swells and releases the cytochrome c into the cytosol. Excess cytochrome c in the cytosol leads to cellular apostosis, see Iverson, S. L. et al. The cardiolipin-cytochrome c interaction and the mitochondrial regulation of apoptosis, ARCH. BIOCHEM. BIOPHYS. 423:37-46 (2003).
Moreover, mitochondrial dysfunction and cell death may ultimately lead to multiple organ failure despite ef resuscitative efforts or supplemental oxygen supply, see Cairns, Charles M D, Rude Unhinging of the Machinery of Life: Metabolic approaches to hemorrhagic Shock, CURRENT OPINION IN CRITICAL CARE, 7:437 (2001). Accordingly, there is a need in the art for an antioxidant mimic similar to SOD which scavenges the ROS, thereby reducing oxidative stress. Reduction of oxidative stress delays, even inhibits, physiological conditions that otherwise might occur, such as hypoxia.
Also, there is also a need to improve the permeability of antioxidants' penetration of the cellular membrane. One of the limitations of SOD is that it cannot easily penetrate the cell membrane. However, nitroxide radicals, such as TEMPO (2,2,6,6-tetramethylpiperidine-N-oxyl) and its derivatives, have been shown to penetrate the cell membrane better than SOD. Further, nitroxide radicals like TEMPO prevent the formation of ROS, particularly superoxide, due to their reduction by the mitochondrial electron transport chain to hydroxylamine radical scavengers, see Wipf, P. et al., Mitochondrial targeting of selective electron scavengers: synthesis and biological analysis of hemigramicidin-TEMPO conjugates, J. AM. CHEM. SOC. 127:12460-12461. Accordingly, selective delivery of TEMPO derivatives may lead to a therapeutically beneficial reduction of ROS and may delay or inhibit cell death due to the reduction of oxidative stress on the cell.
This selective delivery may be accomplished by way of a number of different pathways—i.e., a biological or chemical moiety has a specific targeting sequence for penetration of the cell membrane, ultimately being taken up by the mitochondrial membrane. Selective delivery of a nitroxide SOD mimic into the mitochondrial membrane has proven difficult. Accordingly, there is a need in the art for effective selective delivery of TEMPO antioxidant derivatives that specifically target the cell membrane and, more specifically, the mitochondrial membrane to help reduce the ROS species. Said antioxidants also help prevent cellular and mitochondria apoptotic activity which often results due to excessive ROS species, see Kelso et al., Selective Targeting of a Redox-active Ubiquinone to Mitochondria within Cells: Antioxidant and Antiapoptotic Properties, J. OF BIOL. CHEM., 276: 4588 (2001).
U.S. Patent Application 2005/0169904 discloses a conjugate which comprises the following: (i) a mitochondrial membrane-permeant peptide; (ii) a mitochondrial-active agent or compound of interest such as a detectable group or compound, an active mitochondrial protein or peptide, nucleic acids, drug or signaling agent; and, (iii) a mitochondrial targeting sequence linking said mitochondrial membrane-permeant peptide and said active mitochondrial protein or peptide. The targeting sequence of the conjugate is cleaved within the mitochondrial matrix, not within the cellular cytoplasm of a target cell into which said mitochondrial-active agent or compound is to be delivered. Methods of use of these compounds and agents are also disclosed within the publication. A disadvantage of this metholodology is that it requires cleavage of the peptide sequence in order to release the active agent.
U.S. Pat. No. 6,331,532 and US Patent Application 2005/0245487 A1 disclose mitochondrially targeted antioxidant compounds. The compound comprises a lipophilic cation covalently bonded to an antioxidant moiety. Pharmaceutical compositions containing the mitochondrially targeted antioxidant compounds, and methods of therapy or prophylaxis of patients who would benefit from reduced oxidative stress are disclosed. This methodology relies on ionic or lipophilic interactions and is less selective than the present invention.
US Patent Application 2005/0107366 A1 discloses a pharmaceutical composition that is covalently bound to a non-toxic spin trapping compound. Spin trapping compositions generally have been known to be effective in treating a variety of disorders. Spin trapping compounds are molecules that have an unpaired electron (i.e., paramagnetic), form a stable compound or complex with a free radical, and lack cytotoxicity. One example of a spin trapping compound which is provided is TEMPO. These spin trapping compounds, such as TEMPO, provide a unique signal that can be measured by electron spin spectroscopy (ESR). Since an effective mitochondrial-targeting sequence is not used, this approach is not as efficient as the present invention.
TEMPO and its derivatives are antioxidants that have been shown to improve physiologic variables after induced hemmorhagic shock, such as heart rate, systolic blood pressure, acid-base balance, serum antioxidant status, and survival time, see Kentner et al., Early Antioxidant Therapy with TEMPOL during Hemmorhagic Shock Increases Survival in Rats, J. OF TRAUMA® INJURY, INFECTION, AND CRITICAL CARE, 968 (2002). In general, effective levels of administered TEMPO are too high to accomplish therapeutic effects.
Therefore, in spite of the foregoing prior art, there remains a very real need for TEMPO-related compositions and methods for effectively treating a condition that is caused by excessive mitochondrial production of reaction oxygen species (ROS) in the mitochondrial membrane.